Cucurbituril

Cucurbituril or cucurbit[6]uril C₃₆H₃₆N₂₄O₁₂ is a cyclic chemical compound consisting of six units of glycoluril. This compound is appealing to scientists because the molecule has roughly the shape of a hollow tube or Jack-o'-lantern and the cavity has nanometer dimensions (height approximatly 50 nm, outer diameter 40 nm and inner diameter 20 nm).

The name is actually derived from the resemblance of this molecule with a pumpkin of the family of Cucurbitaceae. Cucurbituril is synthesised from urea, glyoxal and formaldehyde. This reaction was discovered in 1905 by Behrend but structure elucidation took until 1981 (Mock). Cucurbituril is a popular rotaxane component. In this supramolecular assembly a thread-like molecule such as hexamethylene diamine molecule fits in the tube and two tetraphenylborate ions it its extremities act as a stopper. Cucurbituril dissolves poorly in just about any solvent but solubility is greatly improved in a solution of potassium hydroxide or in an acidic solution. Explanation: the cavitand forms an inclusion compound with with a potassium ion of a hydronium ion in a host-guest complex.

Synthesis

Glycoluril 3 is an aminal synthesised from urea 1 and glyoxal 2 in a nucleophilic addition. This intermediate is condensed with formaldehyde to the hexamer. In ordinary multifunctional monomers such as 5 a step-growth polymerization would be expected but due to favorable strain and a abundance of hydrogen bonding the hexamer is the only reaction product. Other molecules that share molecular shape with this compound are cyclodextrins and Calixarenes.

Systematic name

Dodecahydro-1H, 4H, 14H, 17H-2, 16:3, 15-dimethano-5H, 6H, 7H, 8H, 9H, 10H, 11H, 12H, 13H, 18H, 19H, 20H, 21H, 22H, 23H, 24H, 25H, 26H-2, 3, 4a, 5a, 6a, 7a, 8a, 9a, 10a, 11a, 12a, 13a, 15, 16, 17a, 18a, 19a, 20a, 21a, 22a, 23a, 24a, 25a, 26a-tetracosaazabispentaleno[1’’’, 6’’’:5’’, 6’’, 7’’]cycloocty[1’’, 2’’, 3’’:3’, 4’]pentaleno(1’, 6’:5, 6, 7)-cycloocta(1, 2, 3-gh:1’, 2’, 3’-g’h’)cycloocta(1, 2, 3-cd:5, 6, 7-c’d’)dipentalene-1, 4, 6, 8, 10, 12, 14, 17, 19, 21, 23, 25-dodecone (So lets stick to Cucurbituril).

References

• Behrend et al Liebigs Ann. Chem. 1905, 339, 1.
• Mono-, Oligo- und Polyrotaxane mit Cucurbituril und gemischte Polyrotaxane mit Cucurbituril und alpha-Cyclodextrin mittels Selbstorganisation Claudia Meschke German language
• US patent 6365734 Patentstorm
• Mock et al. J. Am. Chem. Soc., 1981, 103, 7367.
• Hans-Jürgen Buschmann, Klaus Jansen, Eckhard Schollmeyer Acta Chim. Slov. 1999, 46(3), pp. 405–411 [1]